1. Field of the Invention
The present invention relates generally to an ultrasonic motor including a rotor and a stator. More particularly, the present invention pertains to a stator and a method for manufacturing the same, which improve the rotational torque, output power and rotational efficiency of the rotor.
2. Description of the Related Art
A conventional ultrasonic motor generally includes a metal stator, a rotor disposed on the top surface of the stator and a piezoelectric element (not shown) disposed at the bottom surface of the stator. When an alternating voltage is applied to the piezoelectric element, the stator is caused to vibrate. The vibration of the stator causes the rotor to rotate. FIG. 25 shows such a conventional stator 50. A plurality of slits 51 are radially formed in the top surface of the stator 50. A plurality of projections 52 are formed in the top surface of the stator 50, between adjacent slits 51. When the stator 50 is caused to vibrate, progress waves are generated in the projections 52. Those progress waves drive the rotor efficiently.
In the above-described stator, it is very important to properly set the width of the slit 51 (the gap between two adjacent projections 52). If the width of the slit is relatively wide, the rotor will not rotate efficiently, since the contact area between the rotor and the top surface of the projection 52 is decreased. Therefore, it is preferable to narrowdown the slit width as much as possible, in order to increase the contact area between the rotor and the projections 52.
Generally, the slits 51 are formed by cutting a base body. There are several consecutive cutting steps that need to be performed. As a result, the time required for manufacturing the stator is rather long, and the manufacturing cost is increased. Furthermore, the large scale polishing process (lapping process) is required, in order to flatten the top surfaces of the projections 52 which slidably contact the rotor. Therefore, the manufacture of the stator by cutting a single metal block requires a relatively long manufacturing time. One attempted solution has been to sinter fine metal particle or casting metal substance, in order to shorten the manufacturing time of the stator 50, and to simplify its lapping process of the stator 50.
However, when the projections 52 are formed in the stator 50 by sintering or casting, there is a limitation on the width of the slit. Further, when the stator 50 is manufactured by the sintering process, the stator 50 become porous. In other words, the density of the stator becomes lower than that of the stator which is manufactured by cutting the single metal block. The lower density of the stator causes the vibration to decrease, and consequently the rotor does not operate efficiently.
Japanese Unexamined Patent Publication No. 3-198674 discloses the ultrasonic motor which includes the stator manufactured with a porous sintered body. As illustrated in FIG. 26, the stator includes a plurality projections 52, each of which has a trapezoidal side 52b. Each projection 52 includes surface 52a inclined toward the center of the stator. When the stator having such surfaces is manufactured by the sintering process, a pre-compressed body having the similar shape to the stator should be manufactured by compressing the fine metal particle, prior to sintering.
During the compressing process, a compressing force is applies to the pre-compressed body along the axial direction thereof. Therefore, the surfaces extending in the direction perpendicular to the direction of the compressing force are provided to secure compressing. But, the surfaces (such as inclined surface 52a) inclined in the direction of the compressing force could not be provided to secure compressing. The density of the inclined surface 52a of the pre-compressed body is lowered with respect to the other parts thereof. When the pre-compressed body having parts of differed densities is sintered, the stator will have uneven density teeth. As a result, such a stator cannot drive the rotor effectively.